Splicing for center wing brake light

[nees]

HELP!!!

At the rear of the driver side brake light assembly, there is a harness of 8 or 10 wires that plugs into the light panel. I need to tap into this for a 3rd center wing brake light. I know the brown is ground. I need to know which of the wires to use for the + on the center wing light so it is on with the left,right brake lights, and also gets brighter with them when the brake is engaged.

Thanks a million!